1. Field of the Invention
The present invention relates generally to laser technology for photolithography, and, more particularly, to optimization of extreme ultraviolet (EUV) light production.
2. Description of the Prior Art
The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (EUV) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 110 nanometers (nm). EUV lithography is generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) with one or more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (LPP), the required plasma can be produced by irradiating a target, such as a droplet, stream or cluster of material having the desired spectral line-emitting element, with a laser beam at an irradiation site.
The spectral line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may be mixed or dispersed with another material such as a liquid. This target is delivered to a desired irradiation site (e.g., a primary focal spot) and illuminated by a laser source within an LPP EUV source plasma chamber for plasma initiation and the generation of EUV light. It is necessary for the laser beam, such as from a high power CO2 laser source, to be focused on a position through which the target will pass and timed so as to intersect the target material when it passes through that position in order to hit the target properly to obtain a good plasma, and thus, good EUV light.
Return beam metrology is used with the EUV source to view the process of generating EUV light, for example, viewing and measuring the light reflected from the target as the target is illuminated by the laser source. Such measurements are referred to as Return Beam Diagnostics (RBD). These return beam diagnostics may include measurements of target position and shape, effectiveness of laser source illumination, laser source focus, and the like.
These RBD measurements are made by a sensing device such as a camera, infrared detector, or microbolometer responsive to the wavelength of the laser source. Due to the operating principle of these sensing devices, their exposure to the reflected light should be controlled when measurements are to be made.
One known method of limiting sensing device exposure to the reflected light is through the use of a mechanical interrupter, such as a set of opaque rotating vanes which periodically block the optical path to the sensing device. The geometry and rotating speed of the vanes defines fixed on and off times, with the spacing between the vanes and the rotating speed defining the on time where a clear optical path is provided for the reflected light to reach the sensing device, and the width of the opaque vanes and the rotating speed defining the off time where the reflected light is blocked from reaching the sensing device.
Because the interrupter defines the times where measurements are made, the fixed nature of such an interrupter imposes limitations in its use with the laser source. Interrupter on and off times are not easily changeable on a measurement to measurement basis. Therefore, it is difficult to maintain synchronization between the interrupter and a pulsed laser source under different operating conditions. Synchronization is needed to insure the reflected light reaches the sensor at approximately the center of the exposure period, so that the sensing device is fully illuminated by the reflected light. Measurements taken when the sensing device is partially occluded by a vane will produce erroneous readings.
What is needed, therefore, is an improved way to control the reflected light reaching a sensing device for making return beam diagnostic measurements in an EUV source.